312 THEORY OF THE ORIGIN OF LAKE-BASINS chap. xv. 



plateaus, and valleys, as in the case of Greenland, before 

 described (p. 235), or be confined to the bottoms of great 

 valley's, as now in the higher Alps), must often, by their grind- 

 ing action, produce depressions, in consequence of the different 

 degrees of resistance offered by rocks of unequal hardness. 

 Thus, for example, where quartzose beds of mica-schist alternate 

 with clay-slate, or where trap-dikes, often causing waterfalls in 

 the courses of torrents, cut through sandstone or slate, — these 

 and innumerable other common associations of dissimilar 

 stony compounds must give rise to a very unequal amount 

 of erosion, and consequently to lake-basins on a small scale. 

 But the larger the size of any lake, the more certain it will 

 be to contain within it rocks of every degree of hardness, 

 toughness, and softness ; and if we find a gradual deepening 

 from the head towards the central parts, and a shallowing 

 again from the middle to the lower end, as in several of the 

 great Swiss and Italian lakes, which ai^e thirty or forty miles 

 in length, we require a power capable of acting with a con- 

 siderable degree of uniformity on these masses of varying 

 powers of resistance. 



2dly. Several of the great lakes are by no means in the 

 line of direction which they ought to have taken had they 

 been scooped out by the pressure and onward movement of 

 the extinct glaciers. The Lake of Geneva, for instance, had 

 it been the work of ice, would have been prolonged from the 

 termination of the upper valley of the Ehone towards the 

 Jura, in the direction from F to G of the map, fig. 42, p. 299, 

 instead of running from f to i. 



3dly. It has been ascertained, experimentally, that in a 

 glacier, as in a river, the rate of motion is accelerated or 

 lessened according to the greater or less slope of the ground; 

 also, that the lower strata of ice, like those of water, move 

 more slowly than those above them. In the Lago Maggiore, 

 which is more than 2600 feet deej) (797 metres), the ice, 



